Galaxies are collections of millions to billions of stars gravitationally bound together. Galaxies come in different shapes and sizes, and fall into three broad categories: spiral (shaped like a pinwheel), elliptical (like a ball, sometimes round and sometimes squashed) and irregular, with no obvious shape or structure. Our own Milky Way galaxy is a spiral.

Spiral galaxies are especially dramatic when seen from different orientations, as they have similar proportions to a compact disc! A spiral galaxy viewed edge-on is a long thin line with a bulge at its center; viewed face-on, as with the Whirlpool Galaxy, its sweeping spiral arms become clear. By observing other galaxies, we can learn more about the history and structure of our own Milky Way.

Figure 22: The Whirlpool - visible image. In this Hubble Space Telescope image, horizontal lines fill the spiral arms that are made up of many millions of hot bright stars and the glowing gas of active star-forming regions. The spiral pattern is created by a pressure wave that moves around the galaxy's center, (a bull's-eye texture) triggering the collapse of gas clouds to form the next generation of stars. Tracing the main spiral arms of the Whirlpool Galaxy, you'll find a confetti pattern at eleven o'clock, a small companion galaxy. It is classified as an irregular galaxy because it lacks well-defined structure.

The next three images of the Whirlpool Galaxy are at the same scale as the visible light image.

Figure 23: The Whirlpool - infrared image. In this Spitzer Space Telescope photo, the spiral arms are again clear. Most of the infrared light in the spiral arms comes from huge dust clouds made up of hydrocarbons similar to soot. The infrared glow from the companion galaxy by contrast comes from the light of old, red stars. It contains little dust and gas, and no new stars.

Figure 24: The Whirlpool - X-ray image. In this picture taken by the Chandra X-ray Telescope, a bull's-eye texture represents the centers of the Whirlpool and its companion galaxy. Much of the diffuse glow is from multi-million degree gas heated by supernova remnants, like the Crab or Kepler's Supernova, that pepper the galaxies. The maximum X-ray emission is centered on the core of each galaxy where a giant black hole is residing. As gas and stars approach and fall into the giant black hole, huge amounts of X-rays are generated. The giant black hole at the center of the Whirlpool has a mass of more than a million Suns. Almost every galaxy, including our own Milky Way, has a giant black hole at its center, but we don't yet know why. Throughout the image you can also feel pinpoints of the X-ray emission. These sources are called X-ray binaries, where a neutron star or black hole is closely orbiting a normal star and feeding off the star's atmosphere. As the gas tumbles toward the neutron star or black hole, it heats up and radiates X-rays.

Figure 25: The Whirlpool - radio image. Radio waves are generated by electrons caught up in the magnetic fields that weave through the spiral arms of the galaxy. this image was taken by the VLA and maps out the location of the strongest magnetic fields found within the spiral arms and in the nuclei of both galaxies.

Galaxies are in constant motion. Even though they are huge distances apart, gravitational attraction can pull galaxies together at hundreds of kilometers per second. Over millions and billions of years, collisions happen. What if two galaxies, each the size of the Whirlpool, collide?